Eric Roberts of Zero Energy Design suggested that I look at CIBSE Guide J. The Guide states:
“Design temperature has a large influence on the capital cost of building services systems, and some influence on running costs. No single design temperature is given for a particular location; rather, a range from which the designer can select an appropriate design temperature in consultation with the client (bearing in mind the previous sentence).”
It goes on to say….
“For buildings with low thermal inertia, earlier editions of Guide A recommended design temperatures such that, on average, only one day in each heating season had a lower mean temperature. Similarly, for buildings with high inertia, a design temperature was recommended such that only one two-day period in each heating season had a lower mean temperature.”
Thus the Hardiness of Plants data would mean that you are designing to THE worst average day and as a consequence the services plant would be oversized and the costs incurred would be greater than necessary.
Having had a quick scout through Guide J it seems that over eight sites –3C to –6C is the range for which min temps in the UK will, on average, be exceeded less than 1 day per year (based on a data set between 1976–1995.)
For the efficient (and cost effective) sizing of plant check the local Met Office data.